This invention relates to a method of automatically monitoring and controlling purifying baths wherein the content of inorganic carbon (IC) or total organic carbon (TOC) or the sum thereof (total carbon TC) in the aqueous purifying solution is determined as measurement and control parameters. The method is conceived in particular for commercial purifying baths in the metal-processing industry and, for example, in automobile construction. It permits, for example, automatic monitoring of the loading of the purifying bath, in particular with fats and oils, characterised by the parameter TOC, and if necessary the supplementation of the purifying bath or the initiation of other bath treatment measures automatically or in response to an external request. The method has been conceived in particular such that the analysis results are transmitted to a location remote from the purifying bath. Furthermore, it is possible to intervene in the automatic measurement process or initiate the refilling or other bath treatment measures from a: location remote from the purifying bath. The xe2x80x9clocation remote from the purifying bathxe2x80x9d may be situated in a superordinate process control system, in a control center of the plant in which the purifying bath is situated, or also at a location outside the plant.
The purification of metal components prior to further processing thereof constitutes a routine task in the metal-processing industry. The metal components may be contaminated, for example, with temporary coatings which have dissolved away or leached out, pigment dirt, dust, metal rubbings, corrosion protection oils, jointing materials such as adhesive residues, cooling lubricants or deformation agents. Prior to the further processing, in particular prior to a corrosion protection treatment or coating (for example phosphation, chromatization, anodization, reaction with complex fluorides, organic coating etc) or prior to lacquering, these impurities must be removed by means of a suitable purifying solution. Spraying, dipping or combined processes may be used for this purpose.
Industrial purifiers in the metal-processing industry are generally alkaline (pH above 7, for example 9 to 12), but may also be acidic. The basic constituents of alkaline purifiers are alkalis (alkali hydroxides, -carbonates, -silicates, -phosphates, -borates) as well as non-ionic and/or anionic surfactants. As additional auxiliary components, the purifiers frequently and/or anionic surfactants. As additional auxiliary components, the purifiers frequently contain complex-forming agents (gluconates, polyphosphates, salts of amino acids such as ethylene diamine tetraacetate or nitrilotriacetate, salts of phosphonic acids, such as salts of hydroxyethane diphosphonic acid, phosphono-butane tricarboxylic acid or other phosphonic or phosphonocarboxylic acids), corrosion protection means, such as salts of carboxylic acids having 6 to 12 carbon atoms, alkanolamines and foam inhibitors, such as alkoxylates of alcohols having closed end groups and 6 to 16 carbon atoms in the alkyl group. If the purifying baths contain no anionic surfactants, cationic surfactants may also be used. Acidic purifiers contain acids, such as phosphoric acid or sulfuric acid, in place of the alkalis.
As non-ionic surfactants, the purifiers generally contain ethoxylates, propoxylates and/or ethoxylates/propoxylates of alcohols or alkylamines having 6 to 16 carbon atoms in the alkyl group, which may also have closed end groups. Alkylsulfates and alkylsulfonates are widely used as anionic surfactants. Alkylbenzene sulfonates are also encountered, although these are disadvantageous from the environmental standpoint. In particular, cationic alkylammonium compounds containing at least one alkyl group having 8 or more carbon atoms are suitable as cationic surfactants.
As a result of the purifying process, the dirt constituents which have dissolved away from the surfaces accumulate in the purifying solution. Pigment dirt may lead to loading with inorganic carbon. Corrosion protection oils, cooling lubricants or deformation agents, such as drawing grease and/or organic coatings which have dissolved away or leached out or jointing materials, lead to the loading of the purifying solution with total organic carbon. As the majority of this total organic carbon is present in the form of mineral oils, mineral fats, or oils and fats of animal or vegetable origin, it is often referred to in abbreviated form as the xe2x80x9cfat loadingxe2x80x9d of the purifying solution. The majority of such oils and fats are present in emulsified form in the purifying solution. Oils and fats of animal or vegetable origin may, however, be at least partially hydrolysed by an alkaline purifying solution. The hydrolysis products may then also occur in dissolved form in the purifying solution. Having too high a TOC loading of the purifying solution, it is no longer guaranteed that the purifying solution will free the components to be purified of oils and fats to the required extent. Alternatively, the danger exists that oils and fats will be drawn back onto the purified components when these are removed from the purifying solution. Therefore. it is necessary to maintain the fat loading of the purifying solution below a critical maximum value which may depend upon the further use of the purified components and upon the composition of the purifying solution. In the case of a high fat loading, it is possible to increase the surfactant content of the purifying solution in order to increase the fat dissolving capacity of the purifying solution. Alternatively, bath treatment measures are initiated with the goal of reducing the fat loading of the purifying Solution. This is any case necessary at a predetermined maximum limit of the fat loading. In the simplest case, the purifying solution is entirely or partially discarded and replaced or supplemented with fresh purifying solution. However, on account of the waste water thereby produced and due to the need for fresh water, it is endeavoured to separate fats and oils from the purifying solution and to continue to use the purifying solution, optionally supplemented with active ingredients. Suitable devices for this purpose, such as separators or membrane filtration apparatus, are known in tile art.
Previously, the purifying efficiency of a purifying solution was conventionally assessed visually on the basis of the purification result. The plant operating personnel assess the purifying efficiency and implement the required measures, such as bath supplementation or bath renewal. This currently customary method requires that operating personnel remain in the vicinity of the purifying bath at the required monitoring times. The shorter the desired monitoring interval, the greater the demands upon the operating personnel for the visual assessment.